1 Prof. Leonardo Mostarda University of Camerino Distributed Systems – intro Prof. Leonardo Mostarda-- Camerino,

Who am I? zProf. Leonardo Mostarda yBorn in Poggio Bustone 2 yLived for the last 8 years in London

Who am I? zDegree + Ph.D. at University of L’Aquila yCooperating with the Ericsson research lab. zConsultancy: Fiat, Ericson, Nokia, … zPost Doc European Space Agency (ESA): yGALELEO satellite system applied to security zResearch associate: Imperial College London yUbival project with Oxford and Cambridge zProf. Middlesex University London zMy research interests: Security, Distributed Systems and wireless sensor networks yHP and IBM research labs and INRIA are using my system 3

Module Outline (provisional) z INTRODUCTION z ARCHITECTURES z PROCESSES z COMMUNICATION z NAMING z SYNCHRONIZATION z CONSISTENCY AND REPLICATION z FAULT TOLERANCE z SECURITY z DISTRIBUTED OBJECT-BASED SYSTEMS z DISTRIBUTED FILE SYSTEMS z DISTRIBUTED WEB-BASED SYSTEMS z DISTRIBUTED COORDINATION-BASED SYSTEMS

Activity Plans

Outline zBasic definitions zGoals when implementing a distributed system ySharing yTransparency yOpenness yScalability zScalability techniques

Definition of a Distributed System: z“A collection of independent computers that appears to its users as a single coherent system.” Tanenbaum & Van Steen. z“A distributed system is one in which the failure of a computer you didn't even know existed can render your own computer unusable” Leslie Lamport z“A distributed system is an application that executes a collection of protocols to coordinate the actions of multiple processes on a network, such that all components cooperate together to perform a single or small set of related tasks.” Google University Code

Definition of a Distributed System A distributed system is organised as a middleware. The middleware layer extends over multiple machines and offer each application the same interface.

Goals z4 goals you should met to make the implementation of a distributed system worth: ySharing yTransparency yOpenness yScalability Prof. Leonardo Mostarda-- Camerino, 9

Sharing zUsers should be able to access easily remote resources zCost effective: yit is cheaper to share a printer instead of buying one for each user. yCostly resources such as supercomputers and high performance storage systems should be shared zEasier to collaborate: yMail, file sharing, Groupware software

Sharing zDo you see any disadvantages in sharing? SECURITY?

Sharing Is a stand alone computer secure?

Transparency in a Distributed System zA transparent distributed system presents itself to users and applications as if it were running on a single computer. zWe need to hide where the processes and resources are physically distributed across multiple computers zLet us have a look what kinds of transparency we have and whether is always required.

Transparency in a Distributed System Different forms of transparency in a distributed systems (ISO,1995)

Degree of Transparency zCan we always achieve a high degree of transparency?  Replicas updating  Communication delays  Masking a transient server failure

Openness zIn an open distributed systems service are specified according to standard rules that describe both syntax and semantic. yInterface Description Language (IDL) for syntax yNatural language for semantic zSpecifications should be complete and neutral in order to allow Interoperability and Portability zflexibility is also important that is how easy is to add and replace components Prof. Leonardo Mostarda-- Camerino, 16

Scalability z“A distributed system is scalable when it improves is performance after adding resources” z“Scalability is the ability of a system, network, or process, to handle growing amounts of work in a graceful manner or its ability to be enlarged to accommodate that growth” zScalability is very hard to achieve

Scalability Problems DNS

Scaling Techniques (1): hiding communication The difference between letting (a) a server or (b) a client check forms as they are being filled.

Scaling Techniques (2): distribution An example of dividing the DNS name space into zones.

Scaling Techniques (3): replication zCache zReplica Any problem? Inconsistency

Checking the learning outcomes zDefine a distributed systems zDefine ySharing yTransparency yOpenness yScalability zWhat is the difference between caching and replica?

Summary zBasic definitions zGoals when implementing a distributed system ySharing yTransparency yOpenness yScalability zScalability techniques

Questions? Prof. Leonardo Mostarda-- Camerino, 24

15 minutes pause Prof. Leonardo Mostarda-- Camerino, 25